1. Technical Field
The present invention relates to a method for forming a back-side illuminated image sensor. It also relates to a sensor formed according to this method.
2. Discussion of the Related Art
FIG. 1 is a cross-section view schematically and partially showing a back-side illuminated image sensor 1, formed inside and on top of a thinned semiconductor substrate 3. In this example, substrate 3 is of type P. Its thickness for example ranges between 1 and 10 μm. On the rear surface side of the substrate, a heavily-doped P-type strip 5 extends from the rear surface of substrate 3 and across a thickness approximately ranging from 10 to 100 nm. Strip 5 is itself coated with a heavily-doped N-type strip 6 of similar thickness. Insulation regions 7 extend from the front surface of the substrate and perpendicularly to this surface and form partitions delimiting, in top view (not shown), a plurality of rectangular substrate portions 3a and 3b. Each region 3a comprises at least one photodiode and may comprise charge transfer devices (not shown), corresponding to a sensor pixel, and each region 3b comprises one or several control transistors (not shown). The front surface of the substrate is coated with a stack 9 of insulating and conductive layers in which various interconnections of the sensor are formed. On the rear surface side, a thin insulating layer 11, for example, made of silicon oxide, covers strip 6. Layer 11 is itself coated with an antireflection layer 13, for example formed of a stack of several transparent dielectric layers of different indexes. Antireflection layer 13 is topped with juxtaposed color filters, altogether forming a filtering layer 15. In the shown example, a first pixel is topped with a green filter (G) and a second neighboring pixel is topped with a blue filter (B). Microlenses 17 are formed on top of filtering layer 15, in front of substrate portions 3a containing the photodiodes.
Regions 7 have the function of insulating substrate portions 3a and 3b from one another. They especially enable avoiding that electrons generated in a given substrate portion 3a, due to the illumination of this substrate portion, are collected by a photodiode of another substrate portion.
Strips 5 and 6 form a junction which enables to limit so-called dark currents. Such parasitic currents are due to the spontaneous random generation of electron-hole pairs at the level of certain defects of the crystal structure of the substrate. They are capable of appearing and of being collected by photodiodes, even in the absence of any illumination of the sensor, and thus disturb the sensor operation. In particular, on the rear surface side of the sensor (in layer 6), at the interface between the substrate and insulating layer 11, crystal defects created at the time when the substrate is thinned down are capable of generating dark currents. The PN junction formed on the rear surface side of the substrate enables to block these currents, so that parasitic electrons are not collected by the photodiodes (the junction is not conductive from layer 6 to substrate 3).
To form such a sensor, it is started from a substrate of standard thickness, for example, with a thickness approximately ranging from 600 to 800 μm. Regions 7 are formed from the front surface of the substrate. They, for example, are implanted regions of same conductivity type as the substrate but of higher doping level, or deep trenches filled with silicon oxide. The photodiodes and the various control transistors of the sensor are also formed from the front surface of the substrate, inside and on top of substrate portions 3a and 3b. After the forming of regions 7 and of the various sensor components, the front surface of the substrate is coated with interconnection stack 9, and a holding handle (not shown) is bonded to the upper surface of this stack. The substrate is then thinned to obtain the desired thickness. It should be noted that lateral insulation regions 7 may extend all the way to the rear surface of the thinned substrate or down to an intermediary depth. After the thinning, strip 5 is formed by implantation of dopant elements of same conductivity type as the substrate, from the thinned surface (rear surface) of the substrate. Then, strip 6 is formed above strip 5, by implantation of dopant elements of a conductivity type opposite to that of the substrate. Insulating layer 11, antireflection layer 13, filtering layer 15, and microlenses 17, are then successively formed on the rear surface side of the substrate.
A disadvantage of this type of sensor is the need to provide, after the thinning step, two successive steps of implantation of dopant elements from the rear surface of the substrate. At this stage of the manufacturing, the front surface of the substrate is coated with stack 9, especially comprising metal layers, for example, made of copper. There thus is a risk of contamination of the implantation equipment by the interconnection metals. In practice, this forces to use implantation equipment specifically dedicated to the forming of strips 5 and 6, separate from the equipment already provided to perform implantations from the front surface of the substrate.
Further, after each implantation, an anneal at a high temperature, for example of at least 650° C., is necessary to activate the dopant elements. A standard anneal, comprising homogeneously heating the sensor assembly, is not adapted since it would result in damaging the components already formed on the front surface side. A laser surface anneal of the rear surface, enabling to strongly raise the rear surface temperature while maintaining an acceptable temperature in the upper portion of the substrate, is thus provided. Such an anneal has the disadvantage of being particularly expensive to implement, and requires specific equipment.
Further, such a manufacturing process comprising an implantation followed by a laser anneal does not enable to obtain a strip 6 having a thickness lower than from 10 to 100 nm. This results in an alteration of the sensor sensitivity, especially for wavelengths in the blue or ultraviolet ranges, for which the photons are absorbed by very small silicon thicknesses. Indeed, when strip 6 is too thick, photons may be absorbed in this strip. The electrons generated during this absorption then have the same fate as the parasitic electrons of dark currents, that is, they are trapped by the PN junction formed between strips 5 and 6, and are thus not collected by a photodiode.